In a coherent optical communication system, optical signals are used to carry data from a transmitter to a receiver. The channel between the transmitter and the receiver may introduce jitter into the data. Jitter refers to a variation in the delay of received data symbols. Because of impairments introduced by the channel, the delay between the received data symbols may vary, instead of remaining constant. Therefore, clock recovery may be performed at the receiver in order to generate a clocking signal such that the jitter is tracked and compensated for in the received signal. Clock recovery is also called timing recovery.
Clock recovery may be implemented using a phase locked loop at the receiver. To implement the phase locked loop, a timing error value is computed from the received signal. The timing error value may be used to correct for timing by providing an appropriately scaled correction signal to a voltage controlled oscillator (VCO) to try to ensure a correct frequency and a correction for the timing phase offset through digital techniques. Computing the timing error value is called performing timing error detection, and different methods for performing timing error detection are possible. One timing error detection method is the Godard method, which is disclosed in the following reference: Godard, D. (1978), Passband timing recovery in an all-digital modem receiver, IEEE Transactions on Communications, 26(5), 517-523. In the Godard method, timing error detection is performed using two narrow rectangular filters over frequencies ±ƒB/2, where ƒB is the baud rate. The baud rate is the transmission rate of the data symbols and is also called the symbol rate. The frequencies ±ƒB/2 are called the clock tones.
A signal carrying data symbols has a finite bandwidth. The excess bandwidth of the signal is the portion of the bandwidth having a frequency magnitude that exceeds ƒB/2. Some timing error detection methods, such as the Godard method, make use of the excess bandwidth of the signal.
If the timing error detection method in the receiver becomes ineffective or fails, then clock recovery may fail.